This invention concerns an improved method for sealing liquid ring-type vacuum pumps used to maintain reduced pressure during distillation of crude maleic anhydride.
Maleic anhydride is conventionally produced by the vapor phase oxidation of hydrocarbon feedstocks. Crude anhydride can be recovered from the oxidation product by absorption into an organic solvent followed by reduced pressure distillation of the anhydride-rich absorbent. The distilled crude maleic anhydride is taken overhead and the anhydride-lean absorbent bottoms are recycled to the absorber. A portion of the crude maleic anhydride is recycled to the distillation vessel and the balance is charged to a lights stripper, also operated at reduced pressure. In the lights stripper low boiling contaminants are distilled overhead. Some maleic anhydride is carried over with the low boiling impurities. Accordingly, as used herein, the term "anhydride distillation system" includes the distillation of anhydride-rich absorbent to obtain a crude anhydride product and the distillation of the crude product to strip low-boiling contaminants.
In conventional process designs, the anhydride distillation system is maintained at reduced-pressure by a vacuum system which includes a vacuum pump or pumps connected to the distillation system through condensers. Vacuum pumps are preferred because when steam jets are used, sufficient water travels back up the feed line to hydrolyze carried over maleic anhydride to solid, insoluble acid, which then can cause severe plugging problems. In addition, any product passing to the jets is lost to further product recovery. Thus, vacuum pumps connected through condensers are preferred over steam jets.
In spite of the use of condensers in the vacuum system, some organic material may collect and foul some moving parts of reciprocating type vacuum pumps. In addition, unless after condensers are provided, maleic anhydride vapor discharged from reciprocating-type vacuum pumps would be lost from the recovered product. Even in liquid ring seal type vacuum pumps, recovered product may foul the sealant if the sealant is not properly selected to provide adequate solubility for absorbed products and is not continuously treated to remove them. Accordingly, it would be advantageous to provide a method which minimizes pump fouling and at the same time increases recovery of product.